Solar has really come down in price since the last time I looked. We wondered if we could start powering our shop off of solar. A good project would be to see if we can build a small scale solar array which would generate power back to the shop and keep it under $1/watt. This would have to include everything from the solar cells to getting usable power out the other end. So we started searching around.

Solar Cell Selection

We went to our favorite buying spot (eBay) and did some looking around to see how much we could find some solar cells for. We found that there are production error cells out there that have some blemishes or chipped edges and you can pick these up for dirt cheap….about 25 cents a watt. So we grabbed 2KG worth (they sell them by weight) which will allow us to make about a 500 watt array. The cells are 3.25″x6″ and are VERY FRAGILE! Who knew these things would break if you breathe on them too hard. They are 0.5 VDC with a maximum 3.6 Ampere output meaning they have a max output of 1.8 watts. Since some of them are chipped or blemished we need to figure we will get less than perfect output from them.

Solar Panel Size and Voltage Choice

We had 9 of the same old windows laying around the shop and they would work really good for about a 50 watt panel each. Wired together they should make a 450 watt array. Really any window panel size will do. Since we are making this on a budget, free windows will definitely help. Making common voltage solar panels helps in wiring them together so they are all working equally. You will lose some voltage from the diode that has to be installed on each panel, but shoot for 12, 18, 24, or 48 volt output panels or arrays.

Electrical Grid Tie Inverter Selection

What exactly is a Grid Tie Inverter and why is that what we want to use? It connects to your existing household AC outlet on on side, and your wind turbine on the other. It converts the DC wind power to an AC pure sine-wave matching the phase of the grid. Since the electricity is being generated on the house side of the power meter, the generated power will actually slow down your power meter or bring it to a stop. But if you are lucky enough to have an old dial turning style power meter AND are generating more power than you are using, it is possible to spin your electrical meter BACKWARDS! I didn’t believe it either until we tried it and it worked.

Grid tie inverters have come down in price and gotten pretty good recently. We expected to pay about 25 cents per watt in this part of the build. They are usually stackable too, which means as your system grows, you can just add another grid tie inverter and they will all just work in tandem.

This is the grid tie inverter we got. It was $138 (27.6 cents per watt) and again we sourced this from eBay.

Assembly and Testing

Here you can see we wired 11 cells in series for each row (5.5VDC). The cells vary voltage from top to bottom and vary current side to side. Meaning if you broke a cell in half 3×3 you would still measure full voltage and have half the amp output. We made 2 full rows, and a third row of cells we broke in half like we just mentioned. Edit: As pointed out by Hack a Day commenter Mark, we effectively limited the current of the entire panel by doing this. This can also lead to overheating in the half panels. At the time, our main concern was getting the voltage high enough for the inverter. For the next set of panels we will be using only full cells. So 3 rows of 5.5VDC = 16.5VDC, then minus the diode voltage drop and we should see around 15-17 Volts Output in operation. We plan on putting 3 panels in series and each series in parallel making a 45-51 Volt (or 48V standard) array.

It makes it easier if you pre-wire a lot of cells before you start. Here you can see we started making piles of them. It really goes quick and is pretty easy to get the hang of once you do a few. The trick is to get a soldering iron that can get hot enough to make it so you don’t have to spend a lot of time on any part of the panel for too long. It took us a few hours to build a 50 watt panel. That time involved will play a part in how big of a system you want to start with.

The cells get wired together easily by running “tabbing” wire down them after applying a flux pen. This make soldering to the solar cells painfully easy. Here you can see all 33 cells wired in series. With each full 3×6 cell theoretically capable of producing 1.8 watts and the halves at 0.9 watts this would be a maximum of 49.5 watts. We’ll round and call this a 50 watt panel.

Once all the cells are wired in place, we tape everything down and give it one last electrical test. This will temporarily hold it in place while we pour a two part polyester resin epoxy over it all to lock it in place.

Here is the epoxy sitting to harden up. When we made this we used old epoxy in cool temps and it came back to bite us. Ours peeled up and ended up taking so long to set up that it leaked through our masking tape and got under some of our panels. We figure we now need fresh epoxy and use about 1/10th of a gallon or less for this size panel. A little goes a long way and really this is just to give it a thin seal and lock everything in place.

Results and Thoughts

Here is the breakdown of our costs:

$125 Solar 3×6 cells creating 500 watts worth
$0 Windows
$50 Gallon of Epoxy
$25~$50 Tabbing wire and diodes
$138 Grid Tie Inverter
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So for roughly $375 we have built a Solar Array capable of producing 35-50 watts in it’s current state, with a maximum of 450 watts if we can get full sun. If we did this again we could get the price down even lower, but for about 75 cents per watt it would only take a few years for us to pay this system off.

The next step on this project is to finish the rest of the panels and tie them all in together and see some long term power generation to get an average for a year. Our first panel puts out about 35 watts in full sun, so we have some improvements to do for the next one.

Let the government pay for 30% of your alternative energy generation project!

As we were digging around with this project I want to make you aware that there are some really good government grants (AS IN FREE MONEY!!!) for those that take advantage of them. As always consult with a professional before deciding to spend any money and never take my word for it. But for what it is worth this website has a lot of good information about getting financial assistance in building alternative energy sources. Check out http://www.dsireusa.org/ to find out if your state has any local policies. As of the writing of this, as long as you START your build before the end of 2011 the Feds will pay for 30% of your project if it is under a commercial business. There are LOTS of grants and loan programs out there to help pay for the costs of installing these systems and now seems like a great time to do it. Earth will love you for it too.

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[…] another look at solar power and thought that the pricing had come down enough for them to consider powering their entire shop via the sun. Cheaper or not, they still didn’t want to pay retail for solar panels, so they decided to […]